While a surface emitting laser device (in particular, a vertical resonator type surface emitting laser is referred to as a VCSEL) can emit a laser beam in a direction perpendicular to the thickness direction of a semiconductor substrate, the device is also recognized to be comparatively easier to be two-dimensionally arrayed.
When the surface emitting laser devices are two-dimensionally arrayed, a parallel processing is enabled by multiple beams to be emitted and, various industrial applications are expected with the view of increasing the density and the speed. When the surface emitting laser array is used as, for instance, an exposure light source of an electrophotographic printer, the printing speed can be increased by the parallel processing of a printing process with use of the multiple beams. In such a printing process of the electrophotography, the quantity of light necessary for exposing a photosensitive drum to light is needed, while at the same time a stable and fine laser spot is necessary to be formed on a photosensitive drum.
Accordingly, the laser beam is required to have both amount and quality, which are respectively such a high-power operation as to satisfy the necessary quantity of light and a single transverse mode operation in a single-peak beam shape.
In the surface emitting laser, an active layer generally has an extremely thin thickness (0.1 μm or less) compared to that of a resonator length (approximately 1 to 2 μm), so that structurally a high-power operation cannot be obtained. Then, a structure that a plurality of active layers are arranged in the resonator, for instance, a periodic gain structure (PGS) in which the active layers are periodically arranged in the antinodes of a standing wave, is adopted as needed.
Thereby, a confinement factor increases, and the output is estimated to increase.
However, in this PGS structure, an i layer in a p-i-n junction theoretically becomes thick. Furthermore, the mobility of a positive hole is smaller than that of an electron. From the reasons, the distribution of the electron and the positive hole is formed in a lengthwise direction of the i layer, and the obtained effect has not reached an expected level.
From the above reasons, a method as in Japanese Patent Application Laid-Open No. 2001-94209 (the 8th page, FIG. 2) is proposed, which can equally inject carriers to each of the active layers even when a plurality of active layers exist as in the periodic gain structure, by forming a pn junction in an in-plane direction.
In addition, a surface emitting laser which can provide high efficiency and high power by forming a current constriction structure and injecting an electric current into a focused necessary region is achieved. In this method, an AlGaAs layer having a high-Al composition, for instance, a composition of 98% Al, is provided in a multilayer film reflection mirror. Then, the AlGaAs layer is selectively oxidized in a high-temperature water vapor atmosphere to form the current constriction structure having a diameter of an aperture, for instance, of 10 μm or less so that the electric current can be efficiently injected only into the necessary region.